cors: update default configuration
cors: update default configuration

Set_defaults has been added into oslo_middleware. So we use it to
override the configuration defaults. This change has been committed
since https://review.openstack.org/#/c/285368/ has merged.

Change-Id: I75b2ea8febdd1c80f1431cd69d09dd985fd2e51c

StackStorm/mistral,openstack/mistral,StackStorm/mistral,openstack/mistral